The invention relates to a pneumatically reversible ram drilling tool (mole), in particular for the production of underground bores without trenching and for laying lines in the ground.
Ram drilling tools of this type are also used for widening pilot bores and for the destructive replacement of pipes laid in the ground; they have a self-controlling percussion piston which can be moved to and fro in a casing by means of compressed air and which transfers its kinetic energy to the casing in a front dead-point position or at a rear dead-point position. In this way, the tool can be operated in forward travel or in reverse travel and, for example when it encounters an obstacle in the ground, or when a blind bore is being produced, can be moved out of the bore in the ground in a reversing operation.
In order to reverse the tool, for example from forward travel to reverse travel, a reversing system is required, which has the effect of braking the percussion piston in its front dead-point position, essentially without transferring its kinetic energy to the casing, with the aid of the operational air, and also has the effect of displacing the rear dead-point position reverses, which ensures that the percussion piston gives up its kinetic energy, for example, to a rear inner collar of the casing.
Reversing systems of this type are known in various versions. For example, there are reversing systems actuated mechanically with the aid of the compressed-air hose and those actuated pneumatically or spring-pneumatically with the aid of the operational air. They can be provided with a mechanical, pneumatic or spring-pneumatic interlock, in order to prevent inadvertent reversing from forward travel to reverse travel and vice versa.
European laid-open specification 0 484 839 has already disclosed a ram drilling tool whose spring-pneumatic reversing system operates not only with the operational air fed to the percussion-piston chamber but also with control air fed separately. In order to make this possible, the tool has a stationary control pipe mounted at the tail end, which is surrounded by a similarly stationary concentric shorter control-air pipe in such a way that an annular duct results, which is connected to a control-air line. The annular duct between the two pipes is connected via a control-air opening to a reversing chamber between the front part of the control pipe and a control sleeve. The control sleeve is mounted on the control pipe at its front end and such that it can be displaced on the control-air pipe at its rear end, and is moved into the forward travel position with the aid of the control air andxe2x80x94after the reversing chamber has been ventedxe2x80x94is moved into its reverse travel position with the aid of the operational air present in the working chamber of the percussion piston. This takes place counter to the force of a prestressed spring arranged in the reversing chamber.
The disadvantage with this tool is that the control sleeve is reversed from forward travel to reverse travel with the aid of the working air acting on its front end face arranged in the percussion-piston chamber, counter to the action of the prestressed spring arranged in the reversing chamber. The front end face of the control sleeve is relatively large, since the compression spring is arranged in the reversing chamber located between the control pipe and the control sleeve, and the chamber therefore has to be dimensioned accordingly. Since the spring force and the force exerted by the operational compressed air on the control sleeve have to have a balanced relationship with each other, the known reversing system requires a correspondingly or voluminously large spring with regard to the size of that end face of the control sleeve to which the operational compressed air is applied.
On the basis of this prior art, the invention is based on the problem of keeping that area of the control sleeve to which operational compressed air is applied as small as possible, in order in this way to be able to use a correspondingly weakly dimensioned return spring for the control sleeve.
This object is achieved in that, in a ram drilling tool having a control pipe fixed to the casing and entering the working chamber of the percussion piston and having at least one control opening, a control sleeve which can be displaced on the control pipe and a control chamber, connected to a control-air line, between the control pipe and the control sleeve, the front end of this control sleeve essentially terminates flush with a collar at the front end of the control pipe.
The collar serves as a stationary control end for the front travel of the tool, whereas conventional tools often have a front control edge which can be displaced with the control sleeve. This offers the advantage that the collar can perform the sealing with respect to the percussion-piston surface which otherwise would be performed by the control sleeve. In addition, arranging the control edge on the control pipe fixed to the casing offers the advantage that it does not change its position in the event of any stressing or bending of the tool casing, whereas in the case of a front control edge arranged on the control sleeve, displacement can occur as a result of increased friction on the basis of bending or stressing of the casing. Bending occurs in particular when the ram drilling tool is passing through ground formations of different strengths and/or on a curved path. In addition, the collar protects the control sleeve located axially behind it against being displaced by the percussion piston moving towards the rear, and serves as a stop for the control sleeve in its forward-travel position.
Accordingly, the front end face of the control sleeve is shielded by the control-pipe collar against the operational air pressure in the percussion-piston chamber, that is to say, in the front end position of the control sleeve, the operational compressed air acts in the axial direction only on the end of the stationary control pipe, but not on the control sleeve.
The control sleeve is preferably supported axially in the tool casing by means of a prestressed return spring; it is then moved from its forward-travel position into its reverse-travel position with the aid of the control air entering the control chamber, counter to the force of the prestressed return spring, without the operational compressed air being involved in this to any noticeable extent.
The control chamber is preferably located between a rear shoulder of the control pipe and an inner collar of the control sleeve, whose axial movement can be limited by a rear stop shoulder.
In order to supply the control chamber with controlled air, the wall of the control pipe can be provided on the outside with a longitudinal groove which, together with a guide pipe surrounding the control pipe concentrically, forms a control-air duct opening into the control chamber. The control-air duct is connected to a control-air line, which preferably runs through the compressed-air hose via which the tool is supplied with operational air.